Added 3 cans of freon year before last; last year needed 4 cans and system was once so empty clutch would not engage. Got 250,000 miles on this '99 ML320. Figure the seal in the compressor is leaking and the entire unit needs replacement with Chrysler/Chinese/Japanese EBay purchase. Meanwhile, I'll follow the old advice to operate it at least weekly during the cold season to keep the seal wet with oil.
What are the odds the leak is somewhere other than the compressor?

To diagnose a leak, buy freon containing UV detector dye and scan with a UV light. The cans are about $20.
This is probably unneeded in my case as the bottom of my compressor is pretty wet with oil. I assume it is refrigerant oil accumulating as my freon escapes and sunburns the polar bears. Not really, we just call it freon. It's really inefficient but environmentally correct R@#& which required MB to install those huge electric fans in front of your radiator in addition to your belt driven fan.